Multimodal Structure Solution with 19F NMR Crystallography of Spin Singlet Molybdenum Oxyfluorides

Complex crystal structures with subtle atomic-scale details are now routinely solved using complementary tools such as X-ray and/or neutron scattering combined with electron diffraction and imaging. Identifying unambiguous atomic models for oxyfluorides, needed for materials design and structure-property control, is often still a considerable challenge despite their advantageous optical responses and applications in energy storage systems. In this work, NMR crystallography and single-crystal X-ray diffraction are combined for the complete structure solution of three new compounds featuring a rare triangular early transition metal oxyfluoride cluster, [Mo₃O₄F₉]^5-. After framework identification by single-crystal X-ray diffraction, 1D and 2D solid-state ¹⁹F NMR spectroscopy supported by ab initio calculations are used to solve the structures of K₅[Mo₃O₄F₉]·3H₂O (1), K₅[Mo₃O₄F₉]·2H₂O (2), and K₁₆[Mo₃O₄F₉]₂[TiF₆]₃·2H₂O (3) and to assign the nine distinct fluorine sites in the oxyfluoride clusters. Furthermore, ¹⁹F NMR identifies selective fluorine dynamics in K₁₆[Mo₃O₄F₉]₂[TiF₆]₃·2H₂O. These dual scattering and spectroscopy methods are used to demonstrate the generality and sensitivity of ¹⁹F shielding to small changes in bond length, on the order of 0.01 Å or less, even in the presence of hydrogen bonding, metal-metal bonding, and electrostatic interactions. Starting from the structure models, the nature of chemical bonding in the molybdates is explained by molecular orbital theory and electronic structure calculations. The average Mo-Mo distance of 2.505 Å and diamagnetism in 1, 2, and 3 are attributed to a metal-metal bond order of unity along with a 1a²1e⁴ electronic ground state configuration for the [Mo₃O₄F₉]^5- cluster, leading to a rare trimeric spin singlet involving d² Mo⁴⁺ ions. The approach to structure solution and bonding analysis is a powerful strategy for understanding the structures and chemical properties of complex fluorides and oxyfluorides.

A hydrogen-bonded assembly of cucurbit[6]uril and [MoO2Cl2(H2O)2] with catalytic efficacy for the one-pot conversion of olefins to alkoxy products

The reaction of the macrocyclic cavitand cucurbit[6]uril (CB[6]) and the diaqua complex [MoO₂Cl₂(H₂O)₂] in hydrochloric acid solution gave a water insoluble supramolecular compound with the general composition 2[MoO₂Cl₂(H₂O)₂]·CB[6]·xH₂O·yHCl·z(CH₃COCH₃) (2). Single crystal X-ray diffraction (XRD) analysis revealed the presence of barrel-shape supramolecular entities, {CB[6]·10(H₂O)}, aligned in layers which are shifted relative to adjacent layers to form a brick-like pattern. The CB[6]/water hydrogen-bonded entities further engage in intermolecular interactions with water, HCl and [MoO₂Cl₂(H₂O)₂] molecules to form a three-dimensional (3D) framework. Compound 2 was characterised by thermogravimetric analysis (TGA), IR and Raman vibrational spectroscopy, and ¹³C{¹H} CP MAS NMR. The reference complex [MoO₂Cl₂(H₂O)₂]·(diglyme)₂ (1) and compound 2 were studied for the oxidative catalytic conversion of olefins (cis-cyclooctene, cyclohexene and styrene) with aqueous H₂O₂ as oxidant. Using alcohols as solvents, 2 was employed in a one-pot two-stage strategy for converting olefins to alkoxy products, which involves oxidation (with H₂O₂) and acid chemistry. Mechanistic studies were carried out using different intermediates as substrates, and the type of solvent and substrate scope were investigated. The results demonstrated the ability of the CB[6]/Mo^VI supramolecular adduct to function as an acid-oxidation multifunctional catalyst, and its recovery and reuse via relatively simple procedures.

The Cucurbit[n]uril Family

In 1981, the macrocyclic methylene-bridged glycoluril hexamer (CB[6]) was dubbed "cucurbituril" by Mock and co-workers because of its resemblance to the most prominent member of the cucurbitaceae family of plants--the pumpkin. In the intervening years, the fundamental binding properties of CB[6]-high affinity, highly selective, and constrictive binding interactions--have been delineated by the pioneering work of the research groups of Mock, Kim, and Buschmann, and has led to their applications in waste-water remediation, as artificial enzymes, and as molecular switches. More recently, the cucurbit[n]uril family has grown to include homologues (CB[5]-CB[10]), derivatives, congeners, and analogues whose sizes span and exceed the range available with the alpha-, beta-, and gamma-cyclodextrins. Their shapes, solubility, and chemical functionality may now be tailored by synthetic chemistry to play a central role in molecular recognition, self-assembly, and nanotechnology. This Review focuses on the synthesis, recognition properties, and applications of these unique macrocycles.